This invention relates generally to methods and apparatus for manufacturing and handling thin wafers of hard, non-metallic material and, more particularly, to methods and apparatus for manufacturing and handling thin wafers of hard, non-metallic materials having at least one planar surface, such as are used as semiconductor substrates.
Reference is made to commonly owned U.S. applications Ser. No. 039,666 filed Apr. 16, 1987, now abandoned, and Ser. No. 191,682 filed May 9, 1988.
The production of extremely thin wafers or rounds of hard, non-metallic material is required in certain applications. For example, substrates for electronic components are formed from non-metallic, monocrystalline or polycrystalline materials, such as silicon or germanium arsenide, which are quite brittle and which have a Vickers hardness of up to about HV 15000 N/mm.sup.2. The physical characteristics of such materials place great demands on machining processes.
Wafers for semiconductor substrates are conventionally manufactured by first producing a cylindrical bar or "billet" of the substrate material from a molten mass. The bar is then sliced transversely to its longitudinal axis, usually using internal hole or compass saws, to obtain discs whose surfaces are then ground to obtain the semiconductor wafers. However, it is difficult to produce wafers having precisely planar and parallel surfaces using such techniques. Another problem is the safe and efficient removal of the wafers from the manfuacturing apparatus for further processing.
More particularly, the cutting or slicing tool tends to migrate or deviate from its intended path during conventional slicing operations under the influence of the various forces which act on the tool during the processing and due to wear and tear on the tool. The deflection of the tool during slicing results in a non-uniformity in the geometry of the disc and, in particular, the surfaces of the disc produced are neither planar nor parallel to each other. Rather, the surfaces of the disc are formed with a "twist" generally referred to as a "bow" or "warp".
A disc or wafer 120 manufactured according to conventional techniques is illustrated in FIG. 1(a) (in exaggerated form). It is seen from FIGS. 1(b) and 1(d) that even further processing steps cannot correct the "out-of-plane" errors in disc 120. The surfaces 100 and 110 of disc 120 manufactured by slicing from a cylindrical bar in accordance with conventional techniques are slightly bowed as seen in FIG. 1(a) due to deviation of the cutting or slicing tool from its intended path. When the thin disc 120 is clamped by suction onto a planar clamping plate 130 for further processing to correct the bow or warp, the surface 110 engaging the clamping plate 130 becomes planar (FIG. 1(b)) due to the slight elasticity of the material of disc 120 which can be referred to as a workpiece. This elastic deformation, however, sets up a pre-stress in the clamped workpiece. The free surface 100 is then machined by any conventional planing process to a planar surface 100' (FIG. 1(c)) to produce the wafer 140. However, when the wafer 140 is released from clamping plate 130, the surface 110 of the wafer facing the clamping plate 130 assumes its original form as seen in FIG. 1(d) under the effect of the pre-stress set up in the workpiece when it is initially clamped to the plate and since the wafer is extremely thin. This bowing of the surface cannot be corrected in subsequent processing steps. Moreover, even if the wafer 140 is then turned over, clamped to plate 130 with surface 100' being flexed into a planar condition, and surface 110 then planed, a bowing would still exist when the wafer is released. It is seen from the foregoing that although it is possible to obtain a wafer having parallel surfaces, e.g., surfaces 100' and 110 are parallel to each other, it is not possible to obtain precisely planar surfaces according to conventional techniques.
The problem of obtaining precisely planar and parallel wafer surfaces is solved by the technique disclosed in above-mentioned abandoned application Ser. No. 039,666 through a method illustrated in FIG. 2 wherein the slicing and planing steps are integrated.
Referring to FIG. 2, the uneven end face 200a (stage 1) of bar 4 remaining from a previous slicing operation is planed, such as by a grinding process, to a precisely planar condition (stage 2), the new planar face of the bar being designated 200b. It is understood that other planing processes than grinding can be employed, such as milling, turning, and electrolytic and errosive abrasion. A disc or workpiece 24 is then formed (stage 3) by slicing the bar in the conventional manner, such as by using an internal hole or compass saw. The resulting disc 24 thus has an uneven surface 200c (due to deviation of the cutting tool during the slicing operation) and the precisely planar surface 200b. However, since the sliced workpiece has the one precisely planar surface 200b, it can be clamped onto a planar clamping plate without any elastic distortion. Thus, the workpiece 24 is then clamped to a planar clamping plate with its planar reference surface 200b engaging the plate whereupon the opposite surface 200c is then machined to a planar condition 210 parallel to the already planed surface 200b engaging the clamping plate to produce the wafer (stage 4). When the wafer is released from the clamping plate, it no longer elastically deforms since there are no pre-stresses set up in the workpiece when it is initially clamped to the plate. The process is repeated as indicated by arrow 212, i.e., the newly formed end face 200a of the bar is then planed, etc. in the manufacture of additional wafers.
It is immaterial in the manufacturing operation whether the severing or slicing and grinding processes are performed to form a surface perpendicular to the axis of the bar or a surface which is slightly oblique to the bar axis.
The integration of the severing or slicing and grinding operations requires a suitable combination of known types of slicing and grinding machines.
For example, apparatus for performing the integrated slicing and grinding operations may comprise a combination of a compass or internal hole saw and a grinding machine. In this connection, apparatus which can perform the grinding or planing of the end face of the bar or billet as well as the slicing of the disc simultaneously with each other in one operation is desired for purposes of reducing the time required for the operation.
One example of apparatus capable of performing the planing and slicing operations at the same time is illustrated in FIG. 3. To permit grinding or planing during the slicing operation, the working edge of the grinding disc 6 is set back radially by a distance "a" behind the cutting edge 5 of the saw blade. Since the disc or wafer 24 has not as yet been severed from the bar 4 in the zone of engagement of the grinding wheel 6, the conditions for a distortion-free planing of a reference face in accordance with the method described above are satisfied.
It is thus possible using the apparatus shown in FIG. 3 to produce wafers with one precisely planar surface so that, if desired, the second surface of the wafer can be subsequently machined by grinding to a precisely planar condition parallel to the first surface.
However, considerable problems exist in the use of apparatus of the type illustrated in FIG. 3 with respect to handling the wafer produced, i.e., in removing the wafer from the bar for further processing.
In particular, during the last phase of slicing by the saw blade of the compass saw, considerable wafer breakage occurs. Additionally, removal of the severed wafer through the inner hole of the saw blade is often difficult due to the narrow space available and the required arrangement of the grinding tool with respect to the saw blade. The removal of the wafer becomes more difficult as the diameter of the wafer increases.